1. The Field of the Invention
The present invention generally relates to high voltage devices. More particularly, the present invention relates to a system for securing a high voltage cable within an x-ray tube.
2. The Related Technology
X-ray generating devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. For example, such equipment is commonly employed in areas such as medical diagnostic examination, therapeutic radiology, semiconductor fabrication, and materials analysis.
Regardless of the applications in which they are employed, most x-ray generating devices operate in a similar fashion. X-rays are produced in such devices when electrons are emitted, accelerated, then impinged upon a material of a particular composition. This process typically takes place within an x-ray tube located in the x-ray generating device. The x-ray tube generally comprises a vacuum enclosure, a cathode, and an anode. The cathode generally comprises a metallic cathode head and a cathode cup disposed thereon. A rectangular slot formed in the cathode cup typically houses a filament that, when heated via an electrical current, emits a stream of electrons. The cathode is disposed within the vacuum enclosure, as is the anode, which is oriented to receive the electrons emitted by the cathode. The anode, which typically comprises a graphite substrate upon which is disposed a heavy metallic target surface, can be stationary within the vacuum enclosure, or can be rotatably supported by a rotor shaft and a rotor assembly. The rotary anode is typically spun using a stator that is circumferentially disposed about the rotor assembly, and is disposed outside of the vacuum enclosure. The vacuum enclosure may be composed of metal (such as copper), glass, ceramic material, or a combination thereof, and is typically disposed within an outer housing.
In operation, an electric current is supplied to the cathode filament of the x-ray tube, causing it to emit a stream of electrons by thermionic emission. A high voltage potential placed between the cathode and the anode causes the electrons in the electron stream to gain kinetic energy and accelerate toward the target surface located on the anode. Upon striking the target surface, many of the electrons convert their kinetic energy into electromagnetic radiation of very high frequency, i.e., x-rays. The specific frequency of the x-rays produced depends in large part on the type of material used to form the anode target surface. Target surface materials having high atomic numbers (xe2x80x9cZ numbersxe2x80x9d), such as tungsten carbide or TZM (an alloy of titanium, zirconium, and molybdenum) are typically employed. The beam of x-rays produced by the electrons then passes through windows defined in the vacuum enclosure and outer housing. Finally, the x-ray beam is directed to the x-ray subject to be analyzed, such as a medical patient or a material sample.
Several types of x-ray tubes are commonly known in the art. Double-ended x-ray tubes electrically bias both the cathode and the anode with a high negative and high positive voltage, respectively. The voltage applied to the cathode and anode may reach +/xe2x88x9275 kilovolts (xe2x80x9ckVxe2x80x9d) or higher during the operation of a double-ended tube. In contrast, single-ended x-ray tubes electrically bias only the cathode, while maintaining the anode at the housing or ground potential. In such tubes, the cathode may be biased with a voltage of xe2x88x92150 kV or more during tube operation. In either case, a sufficient differential voltage is established between the anode and the cathode to enable electrons produced by the cathode filament to accelerate toward the target surface of the anode.
The high voltage applied to the anode and/or cathode is typically supplied via a high-voltage cable. The high-voltage cable typically comprises a plurality of conductive wires protectively covered by an outer covering. In a single-ended tube, one end of the high-voltage cable is attached at one end to an electric power supply, while the other end is typically inserted into a plug connector sufficient to provide the high voltages needed for x-ray tube operation. The plug connector comprises an outer covering and has electrical contacts disposed at one end for electrically connecting the conductive wires of the high voltage cable to the cathode.
Because of the high voltage present in the x-ray tube during operation, the use of insulating structures supportably connecting the anode and/or cathode to the vacuum enclosure or outer housing is necessary to electrically isolate them from the rest of the tube. These insulating structures are typically composed of an electrically insulative material, such as glass or ceramic, and may comprise a variety of shapes. Regardless of their shape however, the insulating structures must accommodate the reduction in voltage from the high voltage present at the anode and/or cathode to the much lower housing or ground potential typically present at the surface of the vacuum enclosure.
In typical x-ray tubes, a cathode insulating structure comprises a hollow conical shape and is composed of an insulating material such as glass or ceramic. The cathode insulating structure attaches at one end to the housing or vacuum enclosure of the x-ray tube and at the other end to the cathode, which it supports in a position proximate the target surface of the anode as described above. In order to supply the high voltage potential to the cathode, the plug connector of the high-voltage cable is typically disposed within the inner volume defined by the conical cathode insulating structure, where it electrically connects to the cathode. The inner surface of the conical cathode insulating structure defines a frustoconical shape. The outer surface of the plug connector of the high-voltage cable that is disposed within the cathode insulating structure also comprises a frustoconical shape near the end that electrically connects with the cathode. This shape is necessary so as to allow the outer surface of the plug connector to complementarily fit against the inner surface of the cathode insulating structure.
A physically close fit between the outer surface of the plug connector and the inner surface of the cathode insulating structure is necessary in order to avoid electric arcing between the surfaces. If a space develops between the plug connector outer surface and the cathode insulating structure inner surface during tube operation, dangerous electrical arcing may occur, which can damage the highly sensitive components contained within the x-ray tube.
In order to avoid problems associated with electrical arcing between the cathode insulating structure and the high-voltage cable, various assemblies have been used in the past to ensure a tight fit between these two components. For instance, cable clamps have been utilized to secure the high-voltage cable within the inner volume of the cathode insulator. Unfortunately, such clamps have suffered from various setbacks. For instance, it is extremely difficult to ascertain the amount of force that such clamps apply to the high-voltage cable disposed within the cathode insulating structure. If too much force is applied, undue stress is inflicted upon the cathode insulating structure and the high-voltage cable, which may reduce the operating lifetime of one or both of the components. Too little force, on the other hand, opens up the possibility for electrical arcing to occur between the insulating structure and the high-voltage cable which, as explained above, is highly undesirable. Specifically, electrical arcing places an undue amount of electrical stress on the cathode insulating structure and the high-voltage cable, which may affect the performance of the x-ray tube and reduce the operating life of the various components therein.
The inability of known high voltage cable clamp systems to determine the amount of applied force between the high voltage cable and cathode insulating structure is exacerbated by other factors. One of those factors is that the high-voltage cable expands and contracts in response to temperature variations present within the tube during operation. When the tube heats up during operation, the high voltage cable heats up as well, which causes it to expand in size. This expansion normally maintains the close fit between the high voltage cable and the inner surface of the cathode insulating structure. When the tube temperature drops, however, such as in response to cooling provided by the tube cooling system, the high voltage cable contracts in size, which may cause the outer surface of the cable to retract slightly from the inner surface of the cathode insulating structure. Gaps created by this retraction increase the chances of electrical arcing, which, as explored above, is undesirable. The high-voltage cable is more likely to have a gap introduced between it and the cathode insulating structure when the proper amount of compressive force has not been applied by the cable clamp. Thus, application of the proper amount of compressive force upon the portion of the high-voltage cable disposed within the cathode insulating structure is critical to ensure the proper operation of the x-ray tube, and more particularly, to avoid the above-described problems associated with electrical arcing.
Further, prior clamp designs have not allowed for the possibility of a low profile connection between the x-ray tube and the high-voltage cable. Such a low profile connection may be desirable where space immediately surrounding the x-ray tube housing is limited.
A need therefore exists for a system for securing a high-voltage electrical cable to an electrical device, such as an x-ray tube, in a low profile configuration. A further need exists for a clamp system that allows the user to easily determine and/or to modify the amount of compressive force being applied to the high-voltage cable within a cathode insulating structure of an x-ray tube such that the operation of the cathode and the entire tube is optimized. It would also be an advantage for the clamp system to enable easy adjustment of the compressive force on the high-voltage cable as may be needed or desired by the user.
In accordance with the invention as embodied and broadly described herein, the foregoing needs are met by a high voltage cable and clamp system for securely maintaining the high voltage cable within an electrical device, such as an x-ray tube. Embodiments of the present invention are directed to a specialized high voltage cable assembly sized and configured to be received within a conical cathode insulator. A clamp assembly imposes a continuously compressive force on the cable assembly sufficient to maintain a tight fit between the cathode insulator and the high voltage cable assembly, thereby avoiding electrical arcing therebetween during tube operation. The compressive force imposed by the clamp assembly is easily determined and adjusted by the user as needed. The high voltage cable and clamp system has a low profile to minimize the amount of space needed to house the system.
In a first embodiment, the high voltage cable assembly comprises a high voltage cable connected at one end to a high voltage power source and at the other end to a plug connector. The plug connector is adjustably connected to the housing or vacuum enclosure of an x-ray tube and serves to electrically connect the high voltage cable to the cathode. The plug connector preferably comprises first and second sections disposed at a right angle to one another. A shallow, cylindrical cavity is defined on the first section near the vertex of the right angle such that the longitudinal axis of the second section of the plug connector is coaxial with the central axis of the cavity. The outer surface of the second section of the plug connector converges to define a frustoconical shape for complementarily fitting against the inner surface of the x-ray tube cathode insulator, which is also frustoconically shaped. The tip of the second section of the plug connector has electrical contacts for electrically connecting to cathode terminals disposed in the convergent end of the cathode insulator.
A substantial portion of the plug connector is disposed within the clamp assembly of the present invention, which is adjustably attached to the x-ray tube via screws or the like. When attached to the x-ray tube, the clamp assembly defines a conduit in which a substantial portion of the first section of the plug connector is disposed. The clamp assembly also defines a cylindrical volume in communication with the plug connector conduit, and has a central axis that is coaxial with the longitudinal axis of the second section of the plug connector. Disposed at least partially within the cylindrical volume of the clamp assembly is a plunger/spring assembly comprising a solid, cylindrical plunger and a spring. A retention screw extending through the spring is attached at one end to the plunger and slidably attached at the other end to a portion of the clamp body. The retention screw maintains the relative position of each of the plunger/spring assembly components during assembly or disassembly of the high voltage cable and clamp system.
In one embodiment, the plunger extends between the end of the spring disposed at least partially in the cylindrical volume of the clamp body and the cavity defined in the first section of the plug connector. A portion of the plunger fits inside the cavity.
The successful operation of an x-ray tube depends in part upon the integrity of the fit between the high voltage cable plug connector and the cathode insulator. As described above, damaging electrical arcing between the plug connector and the cathode insulator may occur if any gap develops therebetween. Complicating this situation is the fact that the plug connector expands and contracts during tube operation in response both to the heat created when x-rays are produced within the tube and to the cooling operations designed to remove that heat. The expansion and contraction of the plug connector increases the chance of gaps between it and the cathode insulator to develop, which increases the possibility for arcing to occur.
The present high voltage cable and clamp assembly prevents such electrical arcing by establishing and maintaining a close fit between the cathode insulator and the high voltage cable plug connector. When properly attached to an x-ray tube, and when the plug connector is relatively cool, such as immediately following the startup of the x-ray tube, the high voltage cable and clamp assembly imparts a sufficient amount of compressive force to the second section of the plug connector so as to maintain it in proximate contact with the inner surface of the cathode insulator. As tube operation continues and the interior tube temperature increases, the temperature of the plug connector rises as well, thus causing the connector to thermally expand. The thermal expansion of the plug connector causes a longitudinal lengthening of the second section of the plug connector, which in turn causes a consequent translation of the plunger, which is partially disposed in the plug connector cavity, up into the cylindrical volume of the clamp assembly. The spring of the plunger/spring assembly reacts to the plunger movement by contracting, thereby maintaining sufficient compressive force upon the second section of the plug connector while allowing the connector to expand. When the plug connector again cools and thermally contracts in size, the spring expands, thereby urging the second section of the plug connector against the inner surface of cathode insulator via the plunger and cavity, thus maintaining a tight fit between the two components.
The compressive force provided by the spring in the plunger/spring assembly may be readily determined via a calibration window defined in a portion of the clamp assembly body. An operator may look through the window to determine the relative position of the plunger, and may adjust the position of the clamp body relative the outer portion of the x-ray tube to which it is attached by adjusting the several screws that affix the clamp body to the tube. The calibration window may be configured with a scale to quantitatively indicate the compressive force being imposed upon the plug connector by the plunger/spring assembly. In this way, an operator may accurately determine the compressive force of the cable and clamp system at any point during the operation of the tube simply by inspecting the position of the plunger through the calibration window.
In a second embodiment, the spring of the plunger/spring assembly comprises a plurality of Belleville-type spring washers disposed in a stacked configuration within the cylindrical volume of the clamp body.
In a third embodiment, an even more compact high voltage cable and clamp system is disclosed wherein the springs of the plunger/spring assembly comprise a plurality of spring washers that are at least partially disposed in the cavity of the first section of the high voltage cable plug connector. The plunger has a shape distinct from that of the other embodiments. Also, the relative positions of the plunger and spring are reversed in this embodiment such that a portion of the spring is disposed in the cavity of the plug connector while the plunger is disposed at least partially within the cylindrical volume of the clamp body. The clamp body has a lower profile, making the cable and clamp system of this embodiment especially useful where little free space is available around the x-ray tube.
The foregoing features of the present high voltage cable and clamp system enable, among other things, a high voltage cable to be secured within an x-ray tube utilizing reliable and inexpensive components. The design of the cable and clamp system is simple and has a low profile, thereby allowing the x-ray tube to be disposed in a relatively smaller space. Also, use of the calibration window enables an operator to readily and accurately determine the amount of compressive force present in the system at an time. Other high voltage electrical devices in addition to x-ray tubes may benefit from the features of the present invention as may be appreciated by those skilled in the art.
These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.